1. Field of the Invention
The present invention relates to a device for analyzing a fluid and to a method of delivering at least one gas mixture especially to a fluid analyzer.
The analyzable fluids may be either gases or liquids. They may be obtained by direct sampling on an industrial process for the purpose of quality control or by sampling a given atmosphere, for example the ambient air, for monitoring or control purposes.
2. Related Art
The analyzers used for measuring small concentrations of chemical species in a fluid sample are sensitive to the characteristics of the pure gases and gas mixtures with which they are supplied. Pure gases or gas mixtures are used to convey the specimen right to the detector and to set a “zero” point during calibration, this being essential in the fluid analysis field. They may also serve to operate the device itself. These pure gases or gas mixtures are called instrumentation gases. For example, these instrumentation gases are helium, nitrogen, air or mixtures such as H2/He, H2/Ar, CH4/Ar, CO2/Ar and H2/N2.
The content of impurities present in these instrumentation gases and the specifications for producing them are parameters that have an influence on the sensitivity of the analyzers and the reproducibility of the analyses.
The levels and nature of the impurities contained in the instrumentation gases enable a quality equivalent to a purity of 99.999% to be achieved. The most frequently guaranteed impurities are moisture and hydrocarbons. Carbon oxides (CO and CO2) and oxygen may also form the subject of guarantees.
Because of ever more stringent regulations, analytical laboratories have to measure ever lower concentrations. The improvement in the performance of analyzers is consequently focused on their detection limit and their precision. The guarantees offered today in the case of instrumentation gases are no longer sufficient to meet these requirements, both in terms of purity and production precision. This is because the impurities present with too high a concentration disturb the background noise of analyzers, thus downgrading their sensitivity. The range of guaranteed impurities may be insufficient and become a source of interference in the measurements. For example, guaranteed impurities are moisture and oxygen, whereas those that are critical for analysis are hydrocarbons, an additional specification enabling interference on the analyses to be limited. Finally, the difference between the deviations in composition from one mixture to another, which is too high compared with the theoretically intended production of the proportions of the various components of the instrumentation gases, is a source of imprecision in the analytical results.
The particular example of a flame ionization detector (FID) as analyzer consists of a flame supplied with a hydrogen/helium mixture and with air, and a collecting plate. The specimen to be analyzed passes through a flame which decomposes the organic molecules and produces ions. These are recovered on a biased electrode and thus produce an electrical signal. An FID is extremely sensitive and provides a wide dynamic range. FIDs are used for detecting hydrocarbons, such as for example methane, ethane or even acetylene. The specimen to be analyzed is mixed beforehand with the combustible instrumentation mixture in a preheated zone. The ions and the electrons formed in the flame are collected and thus enable a current to flow in an external circuit. The current is proportional to the amount of ions, which depends on the concentration of hydrocarbons in the fluids to be analyzed. The current is detected by a suitable electrometer and is displayed on an analog output. Thus, an FID provides a rapid, precise (down to ppb) and continuous reading of the hydrocarbon concentration.
The FID is supplied with two gas mixtures, namely hydrogen/helium (H2/He) in well-defined respective proportions, for example 40% and 60%, and oxygen/nitrogen (O2/N2) in well-defined respective proportions, for example 20% and 80%. The variation in deviations relative to the theoretically intended production of the proportions of the various components from one mixture to another constitutes a source of uncertainty about the results of the analyses carried out using an FID. To improve the reliability of analyses carried out by FID, the H2/He and O2/N2 mixtures, called flame gases, must therefore have steady production precision levels from one mixture to another, thus enabling the influence of this parameter on the measurements to be limited. Furthermore, the content of impurities present in the instrumentation gases is also a factor to be taken into consideration, the more so when small amounts (less than or equal to one part per million) have to be analyzed by FID.
These specifications must therefore be improved in order to ensure reliability of the analyses.
For example in the case of FIDs, the flame gases today are delivered in bottle form with production precision levels between 1 and 2% absolute for the hydrogen content in the case of an H2/He mixture and between 0.5 and 1% absolute for the oxygen content in the case of an O2/N2 mixture. The variation in composition of these mixtures from one bottle to another is a source of error for the end customer. This is because a variation of 2% absolute in the H2 concentration in the H2/He mixture may generate up to a 30% variation in the FID signal obtained for analyzing hydrocarbons. Likewise, a 2% variation in the oxygen content in the O2/N2 mixture supplying the FID generates a 10% variation in the FID signal.
Furthermore, the impurity content is a critical parameter as regards analysis reliability. An impurity concentration of 40 ppb generates a 23% increment in the signal when a zero air sample is analyzed by total FID. However, at the present time the usually guaranteed level of impurities in a bottled mixture is between 50 and 100 ppbv. For example in the case of FIDs, the delivery of bottles of H2/He and O2/N2 mixtures for supplying FIDs therefore has the drawback of generating large uncertainties in the analytical results, which will be all the more critical the lower the contents to be analyzed. The delivery of such bottled mixtures therefore does not guarantee reproducibility over time, either in their composition or in their purity.
This guarantee corresponds to a method of preparing the containers and filling them with mixtures that enables a large number of products to be produced at economically viable costs. The change in requirements in terms of precision and guaranteed levels of impurity means that the production of these bottled mixtures would necessitate modifying the existing processes and plants, something which would incur a significant increase in production costs. Thus, it seems that, for these types of mixtures, the limits in production precision levels achieved by filling centers have today been reached, at the very least for manufacturing a large number of these mixtures. As regards impurities, the guaranteed levels fluctuate depending on the sources used, the process for producing the gas bottles and the method of filling these bottles.